The BBC Trust has launched an investigation into Springwatch host Chris Packham over accusations of breaching impartiality rules.

The Countryside Alliance, which lobbies to promote the interests of rural people and communities, made an official complaint against the TV presenter when he referred to those involved in hunting and shooting as "the nasty brigade" in an article in last October's BBC Wildlife magazine.

In a letter to the Trust, Simon Hart MP and the Alliance's chairman called on the BBC to take action, writing: "We cannot stand by and continue to allow Mr Packham to use the status the BBC has given him to spread propaganda which has a direct impact on the lives of our supporters."

A spokesman for the BBC Trust confirmed that an investigation had been launched into Packham in July which will assess whether any editorial standard guidelines have been breached based on complaints from the Countryside Alliance and the Game and Wildlife Conservation Trust (GWCT).

He said: " The editorial standards committee considered a complaint in July and we expect to publish a decision in September."

Tim Bonner, chief executive of the Countryside Alliance, said the delay in the release of the decision until September was "not in any way reasonable" as it "extends over the Glorious 12th" - the start of the grouse shooting season on August 12.

Packham has received a backlash for recently calling for a ban on driven grouse hunting, with Hampshire Police investigating threats made on social media against Packham for his comments.

A BBC spokesman said that, "no Trust committees meet in August so we expect to publish findings from July's meeting in September".

The presenter has invoked the ire of Sir Ian Botham for his anti-grouse hunting campaigning. He told The Mail On Sunday that he hopes the Trust will tell Packham "to spend more time with the birds, away from the cameras" when they come to a decision in September.

The former cricketer wrote that Packham, who has presented BBC One's Springwatch since 2009, "can not be convincingly neutral" and his comments "drags down confidence in the Corporation among rural communities".

Sir Ian has leant his voice to a campaign called You Forgot The Birds (YFTB), which claims to hold the RSPB - of which Packham is vice-president - to account.

Writing on his blog, Packham said he believed that YFTB was "a concerted attempt by shooting interests to manipulate government policy processes to try and get their demands met on grouse shooting by neutralising the RSPB and people like myself who side with them".